1. Field of the Invention
The present invention relates to a power drive unit for transporting cargo, and particularly to the transportation of cargo within an aircraft.
2. Description of the Prior Art
A large variety of motorized systems for moving cargo are known. Motor driven rollers are employed in some such systems. Cargo and passenger planes in particular often employ series of motor driven power drive units (PDUs) to propel cargo containers and pallets, or unit load devices within the aircraft cargo compartment quickly and efficiently. This configuration can allow for the transportation of cargo from the external loader to the interior of the airplane by a single operator controlling the PDUs.
Cargo within an airplane cargo deck is typically supported by a system of freely rotating floor-mounted conveyance rollers (see FIGS. 1 and 2). Sets or banks of PDUs can be simultaneously elevated from beneath the cargo deck to a level just above the conveyance rollers. Each PDU is a separate electro-mechanical actuator which includes one or more rubber coated wheels or drive rollers. The drive rollers of the elevated PDUs contact and move cargo above the conveyance rollers in the commanded direction upon energization. The movement of cargo depends on the coefficient of friction between the PDU drive rollers and the bottom surface of the cargo, as well as the lifting force generated by the PDU lift mechanism. When the PDUs are deenergized, roller rotation ceases and the cargo stops moving.
Several sets of PDUs can be arranged along a common path of conveyance, and each set can be operated separately, thereby allowing for the. transfer of multiple pieces of cargo. An operator supervising the transportation of cargo into the cargo deck area can guide cargo by means of a joys tick and an on/off switch or similar controls.
PDUs can be damaged when they continue to operate beneath immobilized cargo, a condition known as scrubbing, which can occur when cargo is too heavy or has come upon an obstruction such as a wall within the cargo compartment. Scrubbing can quickly wear away the rubber coating on the rollers necessitating their replacement and can result in damage to the PDU motor.
Cargo container stall sensors integrated within a PDU are used to sense a stalled container and to remove power to the PDU after a predetermined delay to avoid PDU damage. PDUs typically have a manual de-select switch for removing power to the PDUs when a stall condition is sensed. Unfortunately, this de-select switch is often not used properly by operators, who are focused on loading cargo rather than protecting PDUs. Thus, damage to PD Us when scrubbing conditions occur is a common problem.
Problems associated with scrubbing are addressed in U.S. Pat. No. 5,661,384. Experiments have shown that when the temperature of the PDU motor surpasses a certain level, it is indicative of a stalled or jammed condition. The ""384 patent describes a PDU that determines when cargo is jammed by monitoring the temperature of the motor and removing power to the unit when a temperature limit is exceeded. This arrangement, while a significant advantage, is subject to error during, for example, extreme environmental conditions.
Other known stall sensors employ protruding mechanical wheels which are depressed when a payload is positioned over a PDU. This type of system can be a burden to maintain.
Moreover, the extended wheels are not protected from oddly shaped cargo and are readily susceptible to damage.
U.S. Pat. No. 5,568,858 describes a system for detecting a stall condition that employs a sensor driven by the movement of the cargo itself. When overlying cargo is jammed, the sensor does not move, and this absence of motion is sensed electronically. Power to the PDU is then interrupted. This solution is not entirely satisfactory because of the unevenness of the bottom surface of the cargo. In other words, variations in the cargo surface can cause erroneous stall signals. Complex additional sensors may be required to overcome this problem.
In view of the above, it should be appreciated that there is a continuing need for a cargo stall sensor for use with a PDU with improved reliability, and durability. There is also a need for a PDU having a cargo stall sensor that is relatively inexpensive to manufacture and easy to maintain. The present invention satisfies these and other needs and provides further advantages that will be apparent from the description below.
The present invention is embodied in a PDU having drive roller elements that transport cargo and a sensor that effectively detects movement or immobilization of the cargo. Thus, the invention is configured to remove power from the motor driven roller elements when cargo is jammed or otherwise immobilized, thereby preventing damage to the motor and the drive roller elements of the PDU.
The PDU comprises a motor, an output shaft driven by the motor, at least one drive roller element mounted on the output shaft so that the drive roller element is driven through the shaft by the motor, a sensor also driven through the output shaft by a drag device that conditionally permits relative rotation between the sensor and the output shaft, and a motion detector that detects rotation of the sensor.
In one embodiment of this invention, the stall sensor is mounted on the output shaft. The sensor is configured as a wheel, that can carry one or more magnets. When there is no load (cargo) positioned over the PDU, the stall sensor, coupled through the drag device to the drive roller element, rotates at the same speed as the drive roller element. A motion detector, such as a Hall effect sensor, senses the frequency with which the magnet passes and a signal corresponding to that frequency is sent to a logic circuit which causes power to be removed from the PDU when the frequency falls below a predetermined value. This will occur when cargo in contact with the sensor becomes jammed or stopped, causing the frictional forces at the cargo surface to be greater than that at the drag device, thereby stopping sensor rotation.
In a second embodiment of the invention, the stall sensor and at least one of the drive roller elements are frictional connected to each other such that the friction at a drive roller element contact face drives the stall sensor when the friction at the stall sensor is overcome by the friction at the cargo surface. When the frictional force at the cargo surface becomes greater than at the drive roller element contact face because the cargo is jammed, a motion detector detects the relative change in motion between the stall sensor and the drive roller element and sends a signal to a logic circuit and, if a predetermined value is exceeded, the power to the motor is removed.
The invention can be embodied in an airplane with a cargo transportation system comprising a hull with a floor with an array of conveyance rollers mounted on the floor and cargo power drive units for moving cargo on the conveyance rollers. The cargo power. drive unit comprises a translatable frame for retracting and extending the power drive unit into a cargo plane of conveyance, a motor mounted on the frame, an output shaft driven by the motor, at least one drive roller mounted on the shaft and coupled thereto to be driven by the motor to move cargo positioned above the drive roller in a plane of conveyance, a rotatable generally circular wheel frictionally coupled to the output shaft by a drag device for rotation therewith, a plurality of magnets mounted on the wheel, a Hall effect sensor, which can be mounted on the power drive unit to detect the motion of the magnets, and a logic circuit connected to the Hall effect sensor and the motor that removes power to the motor when a signal below a predetermined level is received from the Hall effect sensor.
Other features and advantages of the present invention will become apparent from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The detailed description of the preferred embodiments, set out below to enable one to build and use one particular implementation of the invention, is not intended to limit the claims, but to serve as a particular example thereof.